Composite parts typically consist of filamentary material in a resin matrix. The filamentary material may be unidirectional, or woven, or a combination of both; in fact, most parts are made up of numerous layers of such materials with each layer having a different orientation. The particular orientation of the filamentary material and number of plies will be dictated by the actual design requirements of the part. Typical filamentary materials include: graphite, glass, and KEVLAR. There are two basic types of resins used: thermoset, such as epoxies which cure (undergoes a chemical change) upon heating to forming temperatures, and thermoplastics, such as polyetheretherkeytone, generally called "PEEK", which only melt at their forming temperatures.
The pultrusion process is a simple and cost-effective method of making long, constant, cross-sectional area parts using composite materials. In detail, and in its simplest form, the process involves the "pulling" of resin-impregnated filamentary material through a heated die to simultaneously consolidate and form the impregnated filamentary material as it passes therethrough. A typical process using thermoset resins would begin with the running of numerous strands of filamentary material from individual reels or "tows" through a bath of resin, dissolved in a solvent, and thereafter through a collimating die. The "wetted" filamentary material is then pulled through the die, heated to the curing temperature of the resin, wherein the part is consolidated, formed, and the resin cured prior to exiting the die. Such a method is disclosed in U.S. Pat. No. 3,301,930, "Method of Production of Fiber Reinforced Resin Articles" by L. R. Boggs and U.S. Pat. No. 4,347,287, "Segmented Pultrusions Comprising Continuous Lengths of Fiber Having Selected Areas Along The Lengths Containing Resin Matrix Impregnations" by A. F. Lewis et al. In the latter patent, a method is disclosed for producing a part with cured and uncured segments.
If the part is to be made of numerous layers of filamentary material at various orientations to the "pull" direction (typically designated the X-axis), a "preform" will be necessary. In such cases, the preform is first assembled from tapes or sheets having the desired orientations and passed through the resin bath. Sometimes the preform is assembled from pre-impregnated (often called prepreg) tape, with the tape layers laid up in the proper sequence. Prepreg tape is basically the filamentary material impregnated with the thermoset resin and partially cured to a state wherein it is tacky but easily handled (often called B stage). The preform is pulled through the die, as in the previous example, to form the part. In either case, if the resin is very tacky and/or any of the principle surfaces of the part have filamentary material orientated at large angles to the X-axis, frictional forces will cause the filamentary material in these "off-axis" layers to tend to straighten out and become aligned with the X-axis and/or gather and wrinkle, etc., producing unacceptable parts. In addition, the pull force may become excessive. This problem is solved by prestitching the individual layers of the preform (whether preimpregnated or not) together in a direction perpendicular to the X axis such that the filamentary material orientated along the X axis is used to "pull" the non X axis material through the die. The advantage here is that the preform can still be supplied in rolls so that continuous production can be accomplished. However, while this method works, it requires the time-consuming stitching step.
If the resin is a thermoplastic, the problem of pultruding a preform with off-X-axis filamentary material is even more difficult since, at room temperature, the resin is at full strength and stitching layers of the preform together is all but impossible. In the past, this has limited the off-X-axis orientation of the layers of filamentary material to only a zero angle. Of course, the problem becomes next to impossible when more complex parts, such as "T" cross-sectional parts, are to be manufactured by the pultrusion process. Another approach is to add unidirectional plies over the exterior off-X-axis plies and pultrude an essentially over-sized part. After pultruding, the part is machined to remove the added unidirectional plies. This, of course, is an expensive operation and should be avoided if possible. Thus, what has been needed is a method of pultruding thermoplastic resin matrix preforms that allows large, off-X-axis orientation of the filamentary material.
A system for pultruding tubular composites is disclosed in U.S Pat. No. 4,891,179, "Method of Consolidating composite Articles" by D. G. Peacock, et al. The preform is enclosed in a protective member, for example a glass cloth coated with polytetrafluoroethylene (TEFLON). The cloth can be in the form of half tubes or a flexible type that can be wrapped around the preform. Such a protective member can keep the preform free of longitudinal tension because the Teflon acts as a lubricant preventing the preform from sticking to the die, and the protective member bears the frictional loads. However, the use of conventional composite material having a resin matrix as the protective cover when pultruding high-temperature resin matrix materials, such as PEEK, is impractical. PEEK has a melting temperature in the 700 degree Fahrenheit range (hereinafter referred to as degree F.), well above the useful operating temperature of most other resins. In addition, if the preform is to undergo a large amount of distortion during the pultrusion process, a simple flexible cover will not be adequate.
Thus, it is a primary object of the invention to provide a method of pultruding composite parts.
It is another primary object of the invention to provide a method of pultruding composite parts from preforms which undergo large amounts of deformation during the pultruding process.
It is still another object of the invention to provide a method of pultruding composite parts having a high-melting temperature resin matrix material.
It is a further object of the invention to provide a method of pultruding composite parts having outer plies with large angular variations to the X-axis or pultruding direction.